The present invention, in some embodiments thereof, relates to computer-generated holography and holographic projections and in particular to a method for reducing speckle-noise in holographic projections.
Computer-generated holography (CGH) is a technique generally used to produce a two- or three-dimensional image, frequently including a use of a Spatial Light Modulator (SLM). CGH finds many practical applications in 3D television, 3D visualization (3D images), pico projectors (for example, handheld projectors), biomedical neural interfaces (for example, for artificially stimulating the nervous system), holographic microscopy, and laser micromachining (for example, for manufacturing applications in microtechnology and nanotechnology). Other applications may include optical traps (for example, as optical tweezers to manipulate single molecules, DNA, proteins, enzymes, and the like), and optical cross connects (for example, for high speed communications switching).
The SLM is a device for forming (reconstructing) an optical image corresponding to an electrical or optical input by modulating an incident light. The incident light, which is typically a coherent light such as that produced by a laser, may be phase modulated, amplitude modulated, phase and amplitude modulated, polarization modulated, or direction modulated, or any combination thereof. An example of one type of SLM may be a digital phase-modulating SLM, which may include a plurality of reflective pixels where each pixel is adapted to vary (modulate) a phase of a light wave reflected from it. In a typical use, the phase-modulating SLM is illuminated with the incident light from the coherent light source and reconstructs the optical image (hereinafter may be referred to as “holographic image” or “image”) from an interference pattern created by a diffraction of the modulated light waves reflected by the pixels. A more detailed explanation on the operation of a digital phase-modulating SLM is provided further on herein.
Speckle patterns may appear when reconstructing holographic images, and may contribute to degradation in the quality of the image. Generally referred to as “speckle noise” (when the speckle pattern affects a quality of the image), speckle patterns may be substantially degrading when reconstructing an image using only phase modulation. For convenience hereinafter, “speckle pattern” and “speckle noise” may be used interchangeably. A quantitative measure of the speckle noise may be referred to as “speckle contrast”. In phase-only holography (only phase modulation is used), the SLM may only control the phase of the light reflected, for example, from each pixel and not the amplitude of the reflected light. Thus, the reconstructed image is not uniquely determined, and requires special computational procedures. A common solution is to control only the amplitude of a resultant light field in a discrete set of points in the reconstructed image, while leaving the phase of the resultant light field unspecified (“phase freedom”). A result of controlling the amplitude of the light waves at discrete points is that, between the discrete points, the field fluctuates in accordance with the random phases associated with the neighboring specified points, creating the speckle pattern.
Methods are known in the art for attempting to suppress speckle noise, several of which are identified herein, as follows:
U.S. Pat. No. 4,155,630 “SPECKLE ELIMINATION BY RANDOM SPATIAL PHASE MODULATION” relates to “A process and apparatus for improving image creation in a coherent light imagery system which involves directing diffused laser light onto a mirror having a rocking motion that will cause the reflected rays to sweep a two-dimensional area and focusing the reflected light through a diffuser before collimating same for use in image creation. More particularly, this invention creates the rocking motion by applying a combination of voltages to three independent piezo-electric crystals upon which the mirror is mounted.”
U.S. Pat. No. 4,256,363 “SPECKLE SUPPRESSION OF HOLOGAPHIC MICROSCOPY” relates to “An apparatus for, and a method of, reconstructing and viewing a speckled holographic image through a microscope, with the result that the speckle of the holographic image is significantly reduced, without loss of resolution of the image. A finely-structured and transparent light diffuser is interposed thru the aerial image formed by a hologram or an image formed or relayed by a lens system such as a microscope objective prior to the eyepiece. This diffuser is moved in its plane with a rotary or vibratory motion to suppress the speckle.”
U.S. Pat. No. 6,367,935 “METHOD AND DEVICE FOR ELIMINATING IMAGE SPECKLES IN SCANNING LASER IMAGE PROJECTION” relates to “A method for the elimination of image speckles in a scanning laser projection is suggested, in which a phase hologram is used for dividing the illumination beam of the projector into partial beams. The partial beams are heterodyned again on the image screen within the image element (pixels) to be projected in such a way that differing speckle patterns are formed which average each other out in the eye of the viewer over time and/or space. Thus, a device is provided especially for the laser projection which substantially eliminates or reduces the speckles at the viewer. However, the beam form and the beam density are hardly or not changed.”
US Publication No. 2009/0040527A1 “METHO AND APPARATUS FOR SPECKLE NOISE REDCUTION IN ELECTROMAGNECTIC INTERFERENCE DETECTION” relates to “Interference measurements obtained by comparison of a same beam (i.e. same nominal polarization, intensity, coherence length and wavelength) striking a same region on a sample at a same angle, but having a different beam wavefront upon intersection with the region are shown to provide images with independent coherent speckle noise patterns. Accordingly a plurality of interference measurements with diverse beam wavefronts can be used to identify or reduce coherent speckle noise. Reduction of the coherent speckle noise can be performed by compounding the aligned images. A change in the beam wavefront may be provided by displacing the sample in the direction of the beam between or during the measurements, when the beam is a focused beam (i.e. converging or diverging).”